A common method of encrypting a plaintext message starts by substituting integers for plaintext characters according to some standard alphabet such as ITA2, ITA5, ASCII, or EBCDIC. These integers are then written in binary form to create a first string, or sequence, of 0's and 1's. The first string is modulo 2-added to another, second sequence of 0's and 1's to produce a third sequence of 0's and 1's. The third sequence of 0's and 1's is transmitted as the encrypted message. The sender's object is to make this third string of 0's and 1's appear to be a random sequence of digits in binary form. The intended receiver modulo 2-adds the second sequence to the third sequence to recover the first sequence. Thereafter, the original plaintext message is derived from the standard alphabet that was used, e.g., ITA2, ITA5, ASCII, or EBCDIC. If the second sequence is truly random, an interceptor-attacker will be unable to reproduce the first sequence. Thus, the plaintext message is preserved.
There are a number of problems with this scheme: First, random number strings are a relatively scarce commodity. Second, the receiver must have at hand exactly the same random number sequence the sender used or must be able to reproduce it. Having at hand exactly the same random number sequence the sender used requires the sharing of an enormous amount of key material. The sharing of an enormous amount of key material is impractical. Reproducing exactly the same random number sequence the sender used is impossible.
To avoid these two difficulties, a pseudo-random number generator is commonly employed by both sender and receiver. A pseudo-random number generator is a deterministic machine which, when initialized by a "seed" number, produces a string of digits which appears to be random (by passing various statistical tests). The output of a pseudo-random number generator is periodic, but the period can be made very long. When sender and receiver use pseudo-random number generators to produce the second, key, or encrypting sequence, they start with a common initializing "seed" and synchronize the outputs of their generators. Starting with a common initializing "seed" and synchronizing the outputs of the generators allows a knownplaintext attack in which an interceptor-attacker gains access to plaintext (hence to its binary digit string equivalent in terms of some standard numerical alphabet) and to the corresponding ciphertext. Knowing the digits of the binary plaintext string enables the attacker to reproduce the corresponding pseudo-random number sequence. This frequently allows the attacker to determine the algorithm, initializing "seed," and output sequence of the system's pseudo-random number generator, thus "breaking" the code.
In U.S. Pat. No. 5,113,444, entitled "Random Choice Cipher System and Method," issued to the same inventor as that of the present invention, a string of random digits is added to a string of integers numerically coding the plaintext characters of a message. The summed integer is the body of the cryptogram.
The numerical codings, "numerical synonyms," are randomly chosen from large, randomly dispersed collections of such integers corresponding to the plaintext alphabet characters. The collections of randomly distributed numerical synonyms and their matchings to the characters of a plaintext alphabet comprise a "thesaurus" which must be shared by a sender-receiver pair. So also must be shared a "masking tape," a long string of random digits from which the string of random digits, to be added to the numerical coding of the message, is selected.
The present invention dispenses with the thesaurus, using instead one-time numerical codings for message characters which are themselves extracted from the masking tape.